A typical DF interferometer system locates the direction to a remote transmitter by utilizing the phase difference of the transmitter signal arriving at the individual antennas. DF accuracy of such systems is directly related to DF array aperture size which is determined by the spacing between multiple antennas of an antenna array of the DF system. All other things being equal, larger DF apertures increase direction of arrival (DOA) accuracy. However, simply increasing DF aperture sizes without increasing the number of DF antennas leads to large amplitude correlation side lobes and a real potential for large errors.
A basic problem has been to use such a prior art DF interferometer system to locate the elevation and azimuth of a transmitter, such as a radar transmitter, mounted on an aircraft that is flying at low altitudes above water. Signals transmitted from the aircraft mounted radar received by line of sight and reflected from the surface of the water introduce multi-path error into both azimuth and elevation measurements and degrade the reliability of the estimate of the azimuth and elevation of the directly incident radar signal. In the presence of multi-path effects, the wave fronts of the received signal are distorted so that the gradients of a wave front at a given location may be erratic and inconsistent with respect to the location of the signal source.
In addition, either a phase comparison interferometer or an amplitude comparison direction-finding system over a reflecting surface such as seawater will result in multi-path caused errors in the measurement of signal direction-of-arrival. The occurrence of DOA errors due to multi-path propagation is a function of the transmitter elevation angle, frequency, and the roughness of the surface of the seawater.
U.S. Pat. No. 5,568,394, issued Oct. 22, 1996 and entitled “Interferometry With Multipath Nulling” teaches a method that processes interferometer data to provide for rejection of multi-path signal returns from an emitter and computes an improved estimate of the relative angle between the emitter and an interferometer.
To do this interferometric data is gathered that comprises complex signal amplitudes derived from the emitter at a plurality of emitter angles relative to the interferometer antenna array. The complex signal amplitudes derived at each of the plurality of emitter angles are processed by maximizing a predetermined log likelihood function corresponding to a natural logarithm of a predetermined probability density function at each of the plurality of emitter angles to produce a plurality of maximized log likelihood functions. The improved estimate of relative angle between the emitter and the interferometer is made by selecting the emitter angle corresponding to an optimally maximized log likelihood function. The present method rejects multi path signal returns from an emitter and computes an improved estimate of the angle between the emitter and the interferometer array. The processing method uses a maximum likelihood function that incorporates multi-path statistics so that it is robust against multi-path variability. The present processing method may also be employed to reject radome reflections in radars, particularly those employing antennas having a relatively low radar cross-section.
There are a number of shortcomings to the system taught in U.S. Pat. No. 5,568,394 as compared to the present invention. The patent relates to nulling of multi-path reflections that are stable and repeatable, such as own ship multi-path reflections, including signal blockage. This technique uses previously recorded interferometer data that includes these multi-path effects. It does not and cannot resolve multi-path signals, such as reflected from the surface of the ocean, but identifies the most probable incident wave arrival angle based on previous interferometer array calibrations. There is no mention of polarization diverse antenna arrays and the effect of incident field polarization on the multi-path nulling process.
U.S. Pat. No. 5,457,466, issued Oct. 10, 1995 and entitled “Emitter azimuth and elevation direction finding using only linear interferometer arrays” teaches a direction finding system for using a single linear interferometer array mounted on a moving aircraft to make angle of arrival (AOA) measurements only in sensor coordinates to perform emitter direction finding. True azimuth and elevation to an emitter is determined.
Determining accurate angle-of-arrival (AOA) information for low elevation targets using correlation interferometer direction finding is described in a paper by K. A. Struckman, Resolution Of Low Elevation Radar Targets And Images Using A Shifted Array Correlation Technique, IEEE Antenna Propagation Society International Symposium (1989), pp. 1736-1739, June 1989, Vol. 3.
These linear interferometer arrays generates a direction of arrival (DOA) vector to provide azimuth with no coning error and elevation for location of an emitter. The elevation is derived from phase measurements of signals received from the emitter in a way that allows sequential averaging to reduce azimuth and elevation range estimate errors.
The system generates virtual spatial arrays from the linear array based on the aircraft's six degrees of freedom or motion. Six degrees of freedom refers to the six parameters required to specify the position and orientation of a rigid body. The baselines at different times are assumed to generate AOA cones all having a common origin; the intersection of these cones gives the emitter DOA, from which azimuth elevation range can be derived. The generation and intersection of the AOA cones can be done in seconds, as opposed to the conventional multi-cone AOA approach, bearings only passive ranging, discussed above. Bearings only passive ranging requires that the origin of the cones be separated by some intrinsic flight path length in order to form a triangle, and subtend bearing spread at the emitter.
There are a number of shortcomings to the azimuth—elevation DF system taught in U.S. Pat. No. 5,457,466 as compared to the present invention. The system is designed to operate on a moving aircraft. There is no mention of polarization diverse antenna arrays and the effect of incident field polarization on the direction finding process. There is no mention of multi-path effects on the operation and accuracy of the system. Such multi-path effects are addressed and solved by the applicant's invention. In addition, ambiguous baselines must be resolved.
Thus, there is a need in the prior art for improved DF systems that can compensate for multi-path effects by rejecting the multi-path signals and provide accurate azimuth and elevation measurements for an aircraft with a transmitting radar flying at low altitudes above water.